1. Field of the Invention
The present invention relates to the testing of tab bonds between electrical leads (a.k.a. tabs) and the substrate to which the tabs are attached. The present invention also relates to the testing of semiconductor chip components. Specifically, the present invention relates to a method for the non-destructive testing of the strength of tab bonds by directing a gas at the electrical leads while a current is passed through the electrical leads. During the electrical test of the tab bonds, the semiconductor chip is simultaneously, electrically tested.
2. Description of the Related Art
In the manufacture of semiconductor chips, it is important to assure both the quality and reliability of the chips and their attendant leads. During the manufacture of semiconductor chips, electrical lead wires must be firmly attached to the semiconductor chip so that the chip may be incorporated into still larger components. Usually, the electrical leads are affixed to the semiconductor chip by soldering. Each soldered area, however, presents a potential location where the electrical lead may detach from the semiconductor chip. Wherever the electrical lead disengages from the semiconductor chip, there is an electrical break which renders the semiconductor chip useless.
Traditionally, a visual inspection of the semiconductor chip and electrical leads has been conducted to determine whether the semiconductor chip is properly constructed. During the traditional test, if an inspector found a tab that was either loose or disconnected from the semiconductor chip, the component was rejected from service.
However, the visual inspection method is both slow and unreliable. Although visual inspection may readily detect the absence of electrical leads or tabs, visual inspection is impractical for determining whether tabs are only marginally bonded to the semiconductor chip. In such instances, the inspector may pass a chip with apparently well-constructed bonds, but the bonds may fail later in the manufacturing process because of inadequate tab bond strength. Inadequate bond strength may also lead to failure of the component during use.
Mechanical testing is a known alternative to the visual inspection method. Usually, the mechanical test is accomplished by applying pressure to the electrical lead with a needle or other suitable object. The needle comes into actual physical contact with the electrical lead. However, the mechanical inspection method is as inefficient as the visual inspection technique. Additionally, a mechanical test may destroy bonds which are otherwise acceptable for service because of the physical contact with the electrical leads. Finally, a mechanical test cannot detect marginally bonded electrical leads which may fail during use.
Not only is it necessary to test the strength of the bonds between the electrical leads and the semiconductor chip to assure adequate electrical contact, it is also necessary to test the chip to assure that the semiconductor chip is not itself faulty. To test the semiconductor chip, an electrical current is applied to the chip through the electrical leads. The chip should display a characteristic output signature if it is functioning properly. If defective, the electrical output signal will deviate from the expected, characteristic output signal. As with the case of a defective electrical lead, if a semiconductor chip is defective, it must be rejected from service or repaired and tested again.
The prior art fails to display the individual features which characterize the present invention. For example, U.S. Pat. No. 3,559,054, issued Jan. 26, 1971 to R. W. Bowers, discloses a method for mechanically and electrically testing the quality of joints bonding a chip device to the surface of a substrate of a microminiature module. In effect, a high velocity air blast is directed toward the chip device and the electrical resistance is measured between the chip and its substrate. A faulty bond between the chip and the substrate either will blow the chip off the substrate or will cause a detectable change in resistance between the chip and the substrate. However, this method fails to test either tab bond strength or semiconductor chip operability.
U.S. Pat. No. 3,581,557, issued on Jun. 1, 1971 to J. Drees et al., discloses a pneumatic lead wire tester. The strength of the two bonds between a leadwire, a semiconductor die, and a corresponding semiconductor package lead is tested by passing a gas pulse of selected duration past the leadwire so as to create drag forces on the leadwire. If either bond is defective, the drag forces on the leadwire are sufficient to destroy that bond. The bond then can be reformed or the device can be discarded. The destruction of either bond is detected by visual inspection.
Japanese laid-open patent application 63-250147 of Hiromasa Tsukamoto, laid open on Oct. 18, 1988, discloses yet another method for inspecting the bondability of a wire bond. Tsukamoto teaches that a gas may be directed against the lead wires. If defective, the lead wires will disengage from the substrate. The wires which disengage from the substrate can then be detected visually.
Therefore, in each of the traditional tests for determining the strength of the bonds between the electrical leads and the semiconductor chip, including the examples provided above, the apparatus and/or method fails to provide for simultaneous testing of the semiconductor chip and the tab bonds. The electrical test of the semiconductor chip traditionally has been conducted separately from the test of the electrical leads. Therefore, in each of the prior instances, the electrical test of the semiconductor chip adds a second step to the overall method for approving a semiconductor component for service.
Moreover, each of the prior art methods for testing the bondability of leadwires to their respective substrates fails to provide a method for applying a non-destructive force on the leadwires which directly opposes the bonding force of the tab bonds. In each of the prior art cases, the non-destructive force is merely tangentially applied to the leadwires, as each of the figures in the references depict.